Continuous mode solder jet apparatus

ABSTRACT

A solder jet apparatus is disclosed. The solder jet apparatus is a continuous mode solder jet that includes a blanking system and raster scan system. The use of the raster scan and blanking systems allows for a continuous stream of solder to be placed anywhere on the surface in any desired X-Y plane. This allows for greater accuracy as well as greater product throughput. Additionally, with the raster scan system, repairs to existing soldered surfaces can be quickly and easily performed using a map of the defects for directing the solder to the defects.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/569,215,filed May 11, 2000, now U.S. Pat. No 6,325,271, issued Dec. 4, 2001,which is a continuation of application Ser. No. 09/388,032, filed Sep.1, 1999, now U.S. Pat. No. 6,082,605, issued Jul. 4, 2000, which is acontinuation of application Ser. No. 08/989,578, filed Dec. 12, 1997,now U.S. Pat. No. 5,988,480, issued Nov. 23, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to applying solder to asubstrate and, more particularly, to the selected placement of solderusing a solder jet.

Depositing solder selectively on a substrate is well known. Differenttechniques include stenciling or screening a solder paste onto thesubstrate, using solder balls selectively placed where metal contact isdesired, and chemically vapor depositing the metal onto the surface ofthe substrate. Each one of these methods has advantages anddisadvantages.

The use of a stencil to fabricate a conductive trace pattern on thesurface allows for precise alignment and placement of the solder.Unfortunately, the stencils are expensive to design and produce and theywear out after repeated use. When they wear out, solder seeps throughthe worn stencil areas across those areas where no solder is desired,causing shorts, or no solder is being placed where it is needed, causinga breach or open connection. These areas have to be repaired and ifthese types of conditions are repeated with any type of frequency, thestencil must be replaced with a new stencil. Additionally, stencilsrequire periodic cleaning, which adds another processing step to cleanthe stencil as well as lessens the useful life of the stencil.

The use of solder balls has been a tremendous advance in the art ofelectrically connecting a device to the surface of a printed circuitboard. Solder balls, however, have quality control problems as theircritical dimensions continue to decrease. The ability to produce ballsof the same diameter consistently decreases as the diameter decreases.Thus, for some diameters of solder balls, the range of acceptableproduct can be solder balls having diameters more than twice the desireddiameter. Or, they can have diameters half the size of the desireddiameter. This requires that the tolerances at the surface contact levelof a substrate, such as a semiconductor device, must allow for a solderball having a diameter that is from 50% smaller to 100% larger than thespecified size. Further, working with solder balls is difficult becauseof their size and the methods needed to place them accurately. When theyfail to be placed accurately, or are missing entirely, problems occur inthe resulting assembly of a semiconductor device attached to a substratethat must be corrected. These problems include shorts or opens that mustbe fixed. No easy solution yet exists for repairing missing orimproperly sized solder balls after a semiconductor device has beenmechanically attached in place on a substrate.

Chemical vapor deposition (CVD) allows for precise alignment ofconductive traces and for batch processing. CVD does have limitationshowever. These limitations include being unable to place the packagedirectly on the surface of the printed circuit board (PCB) immediatelyafter depositing the metal on the surface since a cooling step istypically needed. Further, clean conditions are always necessary whenusing CVD, which requires expensive equipment and control. Additionally,when clean conditions do not exist, shorts or opens in assemblies canoccur that need to be repaired once they are discovered.

A new approach to deposit solder on a surface, such as a printed circuitboard (PCB), is to deposit the solder using a solder jet, similar to themanner in which ink jets deposit ink onto paper for printing. The inkjet technology is well established, but due to different problemsassociated with solder, ink jet technology is not directly applicable tosolder jet technology. For example, solder jets use molten melt as aprint agent, whereas ink jets use heated water-based ink. Since theprint agent is metal in solder jets, the viscosities and densities aremuch different as are the operating temperatures. Thus, applying ink jetsolutions to solder jet problems is impractical.

One typical solder jet apparatus has recently been developed by MPMCorporation. The solder jet apparatus takes liquid solder and forms itinto a stream of droplets that have a uniform size and composition. Theformation of the droplets involves generating a consistent pressurecoupled with a vibration force sufficient enough to dislodge the dropsfrom the jet nozzle in a steady state with a uniform size andconsistency. Once the solder droplets are formed, gravity forces themdownward where they impact on the surface of the substrate. The solderdroplets pass through a charging electrode to impart a charge on themetal droplets.

The system operates using a binary control that either allows thedroplets to impact on the surface or to be removed into a dropletcatcher for recycling when no droplets are desired. Since the dropletswere charged at one point, an electric field or pulse can be asserted,causing the droplets to either continue to the surface or to fall intothe catcher. With this system, the exact position of the droplets isknown and never varies. Thus, the substrate must be moved to the desiredgrid for the droplets to impact the area desired to be soldered. Thisresults in a highly inefficient system since the substrate must bestopped for each application of solder to a new location. This alsoinvolves greater mechanical complexity since the table holding thesubstrate, or the solder jet apparatus itself, must be moved and alignedproperly before solder can be deposited.

Accordingly, what is needed is a solder applicator that allows forgreater precision in placing the droplets along with increasedefficiency in product throughput.

SUMMARY OF THE INVENTION

According to the present invention, a solder jet apparatus is disclosed.The solder jet apparatus is a continuous mode solder jet that includes ablanking system and raster scan system. The use of the raster scan andblanking systems allows for a continuous stream of solder to be placedanywhere on the surface in any desired X-Y plane. This allows forgreater accuracy as well as greater product throughput. Additionally,with the raster scan system, repairs to existing soldered surfaces canbe quickly and easily performed using a map of the defects for directingthe solder to the defects.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a solder jet apparatus accordingto the present invention; and

FIG. 2 is a top plan view of a substrate having solder depositedaccording to the solder jet apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

A solder jet apparatus 10 is depicted in the schematic block diagram ofFIG. 1. Solder jet apparatus 10 deposits metal on substrate 12 in theform of solder droplets 14. The solder droplets 14 can be directed in anX-Y plane of deflection using a raster scan and blanking system. Thisallows the solder droplets to be “written” on substrate 12.

The solder droplets 14 are formed from melted metal held in liquid metalreservoir 16. A temperature controller 18 is connected to liquid metalreservoir 16 so that the temperature of the liquid metal held in thereservoir can be kept at a desired temperature that leads to optimumdroplet formation and release. For example, the solder eutectictemperature at the point of release is 190° C. and its temperature atimpact is 183° C. To prevent solder droplets 14 from cooling too rapidlyor from oxidizing, a constant surrounding temperature is provided and,if desired, the apparatus can be placed in a container that is eitherunder vacuum or is filled with an inert gas.

The solder droplets 14 can be formed by the application of a drivingpressure and a sufficient vibration force. The driving pressure can beprovided by pressure inducer 20, which is comprised of a piezoelectriccrystal that is driven by a tuning frequency sufficient pressure tobuild up in liquid metal reservoir 16. The mechanical vibration isgenerated by vibrator 22, which comprises a second piezoelectric crystalthat is driven by another tuning frequency, causing liquid metalreservoir 16 to vibrate. The timing of the pressure and the vibrationsis established so as to produce uniform droplets of the sameconsistency. Once the solder droplets 14 are formed, the vibrationreleases them from liquid metal reservoir 16 and the force of gravitydraws them down at a predictable velocity.

Liquid metal reservoir 16 further includes a solder jet nozzle 23, whichis open and closed via a solenoid 24. The aperture of solder jet nozzle23 is selected with a size sufficient enough to generate the droplets ofa desired size. The solder droplets 14 are formed metal reservoir ofmicron size, ranging from 40-300. When solenoid 24 is activated, iteither closes or opens solder jet nozzle 23.

Solder droplets 14 pass through several zones before either beingdeposited on substrate 12 or recycled back to liquid metal reservoir 16.The first zone is a charging field driven by charge driver 26. Chargedriver 26 causes charge electrodes 28 to generate an electric fieldtherebetween. As solder droplets 14 pass past charge electrodes 28, theyare imparted with an electric charge. With this charge, solder droplets14 can be deflected at later stages as appropriate.

The second zone is a blanking zone that uses blanking electrodes or coil30. The blanking electrodes are activated having sufficient electricfield so as to cause solder droplets 14 to deflect to a catcher 32. Thisis the return function of the scanning function as is described below.Catcher 32 catches the liquid solder and causes the metal to be recycledto liquid metal reservoir 16. This prevents solder droplets 14 fromdepositing on the surface of substrate 12. This blanking can be done ina selective manner so that droplets are deposited in some locations, butnot others. Blanking electrodes or coil 30 are controlled by signalcontroller 34. Signal controller 34 can be a signal processor such as acomputer system. The computer system allows greater control of solderdroplets 14 by programming the blanking electrodes or coil 30 to turn onand off in a desired sequence so as to pattern the substrate with adesired solder pattern. An alternative embodiment can include an air jetsystem if the electrical pulse is insufficient to remove the droplets. Aphoto cell can be located above the air jet system in order to ensureproper timing of electrical pulses or the air pressure.

The third zone is the raster scan system and includes electrostaticdeflection plates or magnetic coil 36. Electrostatic deflection plates36 are charged by signal controller 34 so that solder droplets 14 aredeflected in either the horizontal X-direction or the verticalY-direction, or both. Further, the solder droplets 14 can be held in asteady position in the X-Y plane in order to build up the solder to adesired height. Since the droplet stream now scans in the X-andY-directions, the substrate 12 can now stay stationary throughout thedroplet application process. Signal controller 34 can be programmed toperform a variety of soldering patterns for placing solder droplets 14on substrate 12. For example, a CAD/CAM system programmed with a desiredoutput sends signals to blanking electrodes 30 and to electrostaticdeflection plates 36 to guide the droplet stream in the desired patternof placing droplets in certain locations, but not in others.Additionally, when the “stream” of solder droplets 14 is returned to thebeginning of the horizontal scan, blanking electrodes 30 cause thesolder droplets 14 to deflect to catcher 32 so as not to “write” acrossthe substrate during the return scan. The location of blankingelectrodes 30 and electrostatic deflection plates 36 can be switched, ifdesired.

An electronic light sensor 38, which connects to signal controller 34,is positioned so that the solder droplets 14 pass through the electroniclight sensor 38. Electronic light sensor 38 is used to count the numberof solder droplets 14 passing by. This allows signal controller 34 tomonitor the droplet output and either blank or pass droplets as needed.

FIG. 2 is a top plan view of the surface of substrate 12 as solderdroplets 14 are deposited. A first line 40 scans across the surface,depositing solder droplets 14 in selected positions and leaving blanks42 in the remaining positions. A return scan line 44, which is ghosted,indicates when the stream of droplets is caught by catcher 32 as thestream returns to the beginning of the next line 40. This process isrepeated as often as is necessary with catcher 32 collecting all theblank spots and scan returns. Alternatively, solenoid 24 can beactivated to close solder jet nozzle 23 during the return scan. Thisalso prevents unwanted solder droplets 14 from depositing on the surfaceof substrate 12.

The type of solder used with the solder jet apparatus 10 can include anytype of metal solder such as, for example, 63/37 PbSN, 62/36/2PbSnAa,In/Sn.

The solder jet apparatus 10 can be used for many types of solderapplication. One type of application includes that of applying uniformsolder balls, in the form of solder droplets, to the substrate 12. Thisprovides a universal ball applicator system. Further, the system canrepair particular locations where the solder ball application processhas failed to insert a desired solder ball. In order to repair any andall solder ball defects, a scan of the surface of substrate 12 can beprovided and then a map of the defective areas can be programmed to thesignal controller 34. This allows for a rapid repair of the surface ofsubstrate 12 where solder balls had been omitted. Another application isto pre-tin a location on substrate 12. Pre-tinning is accomplished byapplying one or more droplets to the same location or to apply dropletsin such a manner as to thoroughly cover the surface of substrate 12 or agrid section of substrate 12.

Similar to pre-tinning is pre-plating a board. Pre-plating a boardinvolves applying solder droplets over the entire surface area of theboard to cover it with a metal plate. An exposed portion of the boardcan be selected where desirable. Typically, this area is along the edgeof the board either on one edge, two edges, or all four edges, or can bein the center section of the board. Prior methods of pre-plating a boardresulted in a problem known as “measling.” Measling is where small holesexist in the plating surface that lead to electrical defaults. The useof the solder jet apparatus 10 allows the system to eliminate themeasling locations by applying solder directly to those openings.Additionally, using the pre-plating process provided by solder jetapparatus 10 eliminates measling entirely. Just as pre-plated boards mayhave measling problems, boards that had been stenciled with solder pastehad similar problems. These problems can include openings or gaps in thestenciled design. Again, a map of the surface defects can be ascertainedand then used by the signal controller 34 to make appropriate correctionand repair to those particular problem points. Additionally, large areascan be printed using the X-Y motion of the table in combination with theX-Y slowing of the solder application. Also, the final ball size can bechanged on demand. Further, in prior ball application systems that apply7 balls/sec, the board needs to be moved to a new location. With thisinvention, no relocation time is required, thus reducing processingtime.

While the present invention has been described in terms of certainpreferred embodiments, it is not so limited, and those of ordinary skillin the art will readily recognize and appreciate that many additions,deletions and modifications to the embodiments described herein may bemade without departing from the scope of the invention as hereinafterclaimed.

What is claimed is:
 1. A liquid solder jet apparatus for depositing astream of liquid solder droplets on at least one bond pad of selectedbond pads of at least one semiconductor die of a substrate having asurface having a plurality of locations thereon extending throughoutsaid surface, each location of said plurality of locations on saidsurface having a start point and an endpoint, comprising: a continuousstream generator for producing a stream of liquid metal solder droplets,the liquid metal solder droplets having a uniform size within aconsistent predetermined range; and a stream director for selectivelydirecting said stream of liquid metal solder droplets after beingproduced by said continuous stream generator onto said at least one bondpad of said at least one semiconductor die of said substrate, saidstream director comprising a raster scanner scanning said stream ofliquid metal solder droplets, said raster scanner including: anelectrical charge generator for charging at least a portion of saidliquid metal solder droplets of said stream of liquid metal solderdroplets with an electrical charge; a stream blanking device forintermittently blanking at least some of said liquid metal solderdroplets of said stream of liquid metal solder droplets; and anelectrically charged droplet deflector for deflecting at least oneelectrically charged liquid metal solder droplet of said stream ofliquid metal solder droplets in a first direction and a second directionfor deposition at a location of said plurality of locations extendingthroughout said surface of said substrate when said substrate remainsstationary.
 2. The apparatus according to claim 1, wherein saidcontinuous stream generator further comprises: a reservoir for holdingliquid metal solder; and a temperature controller connected to saidreservoir for maintaining said liquid metal solder in a liquid state. 3.The apparatus according to claim 1, wherein said continuous streamgenerator further comprises: a pressure inducer; and a vibratorconnected to said pressure inducer for causing formation of said streamof liquid metal solder droplets in connection with said pressureinducer.
 4. The apparatus according to claim 3, wherein said pressureinducer comprises a piezoelectric crystal operating at a desiredfrequency.
 5. The apparatus according to claim 3, wherein said vibratorcomprises a piezoelectric crystal operating at a selected frequency toform liquid metal droplets having a size in the range of micron sizedroplets of a liquid metal solder.
 6. The apparatus according to claim1, wherein said continuous stream generator further includes a solderjet nozzle having an aperture producing a consistent range of dropletsof a liquid metal solder for forming said stream of liquid metal solderdroplets.
 7. The apparatus according to claim 6, wherein said continuousstream generator further includes a solenoid connected to said solderjet nozzle.
 8. The apparatus according to claim 1, wherein said streamblanking device at least provides blanking of said at least some of saidstream of liquid metal solder droplets when said steam of liquid metalsolder droplets is positioned between said endpoint of a first locationof said plurality of locations extending throughout said surface of saidsubstrate and said start point of a second location of said plurality oflocations extending throughout said surface of said substrate.
 9. Theapparatus according to claim 1, wherein said stream blanking devicefurther comprises: a deflector field device selectively deflecting atleast one droplet of said stream of liquid metal solder droplets; and adroplet catcher catching said at least one droplet which has beendeflected from said stream of liquid metal solder droplets prior to saidat least one droplet which has been deflected from said stream of liquidsolder droplets being deposited on said at least one bond pad of said atleast one semiconductor die of said substrate.
 10. The apparatusaccording to claim 1, wherein said stream director includes aprogrammable direction controller for determining a direction of saidstream of liquid metal solder droplets.